The use of meter actuated pilot valves for the relayed operation of main flow control valves and the advantages achieved thereby are well known and understood. In systems capable of dispensing a predetermined amount of liquid, wherein a flowmeter actuates a pilot valve to effect closure of the main valve when a predetermined quantity of liquid has been delivered, it is frequently essential particularly when delivery is made at high flow rates to stage the closure of the main control valve. One reason for this is to minimize the shock which would otherwise occur if the main valve were to be suddenly completely closed from a full open, high flow rate position and another reason is to permit effecting a more precise cutoff at the instant a predetermined amount has been delivered.
Various pilot and main valve constructions and their arrangements in such systems have been proposed. In some prior arrangements the pilot valve is mounted on or incorporated within the main valve body and mechanically connected to flowmeter actuated preset mechanism spaced therefrom by linkage. In other prior arrangements solenoids are employed to operate pilot valves which solenoids are energized through meter actuated switching means.
The mechanical connection of a pilot valve to meter actuated preset mechanism when considerably spaced therefrom by linkage introduces the potential for inaccuracies in the precise motion transmission required and the necessary frequent adjustment thereof to maintain an acceptable degree of accuracy has been found troublesome. The solenoid operation of pilot valves substantially increases the cost when staged closure of the main valve is required because multiple solenoid operated valves are required to stage the closure of the main valve. Moreover, solenoid operation of the pilot valves requires a suitable source of electrical power which is not always available.